This invention relates to a physiological signal monitoring system and more particularly to a system which allows a user to determine various types of physiological information and which allows a user to electronically access this information over a communication network.
Various types of instrumentation for monitoring physiological signals are currently available to consumers and health professionals. Specifically, consumers have access to thermometers, weight scales, blood pressure cuffs, blood glucose monitors, urine testing strips and other similar diagnostic technology. In the field of cardiovascular physiological testing, there is currently a wide variety of blood pressure testing equipment which has been developed to determine arterial blood pressure related parameters, namely systolic pressure (maximum blood pressure) and diastolic pressure (minimum blood pressure). It has also been recognized that other parameters such as mean (average) blood pressure during a heart cycle, pulse pressure (the difference between systolic and diastolic pressure) as well as pulse rate and pulse rhythm are also important in assessing patient health.
In an attempt to provide consumers and health professionals with non-invasive blood pressure measuring equipment for patient safety and convenience, photoplethysmograph (PPG) sensors have been utilized within blood pressure testing equipment. PPG sensors are well-known instruments which use light for determining and registering variations in a patient""s blood volume. They can instantaneously track arterial blood volume changes during the cardiac cycle and are used within physiological signs monitoring devices.
One such device is disclosed in U.S. Pat. No. 6,047,203 to Sackner et al. which uses PPG sensors to monitor the physiological signs of the user to identify when adverse health conditions are present within the user and to provide the user with appropriate directions or signals. However, many devices such as this one are only used to determine whether physiological signals indicate the presence of an adverse condition for the user and are not directed to identifying and/or determining accurate estimates of blood pressure and other cardiovascular values for diagnostic purposes.
Since PPG sensors operate non-invasively, efforts have been made to utilize them to determine estimates of mean, systolic and diastolic blood pressure. These devices either estimate mean blood pressure from the mean value of the blood volume pulse, a measure of pulse wave velocity or changes in the volume pulse contour using formulae and calibrated constants. However, these devices have not achieved widespread use due to a lack of accuracy and difficulty of use.
Specifically, the difficulties with estimation of mean, systolic and diastolic blood pressure from the volume pulse contour can be attributed to variability in the amplitude of the volume pulse contour due to volume changes unrelated to blood pressure effects and the nonlinear relationship between volume changes in an arterial vessel and associated pressure changes.
Also, there are measurement and instrumentation difficulties associated with PPG sensors such as the presence of mechanical alterations in the sensor/skin interface (i.e. vibrations and differing pressure), ambient light effects, and changes in the blood volume due to alteration in body position. Without carefully correcting for changes in the blood volume pulse signal that are due to factors other than blood pressure and without using conversion techniques which recognize the nonlinear relationship between arterial vessel volume and pressure, these methods cannot accurately predict blood pressure characteristics using PPG readings alone.
It has long been recognized that blood volume pulse contours change with aging and blood pressure. These changes are largely related to a shift in the occurrence of the aortic reflected wave within the pulse contour. The reflected wave is a complex pulse signal generated by reflections of the pulse wave originating at the heart. The pulse wave travels from the heart along the aorta with branches to the head and the arms, continues along the aorta to the trunk and from there to the legs. At about the level of the kidneys, a significant reflection of the pulse wave originates. The reflected waves from the arms and the legs are rapidly damped, travelling with relatively low amplitude back to the trunk. It is well known that as detected in the upper extremity the reflective wave originating in the abdominal aorta has an onset later than the reflected wave from the upper limbs, has significantly greater amplitude, travels almost without attenuation to the ends of the upper extremity, and has a significant presence in the volume pulse contour obtained from a fingertip, ear or other points on the surface of the body above the aortic origin of the reflecting wave.
By accurately characterizing the timing, amplitude and shape of the abdominal aortic reflected wave, a significant amount of information about aortic compliance, aortic pulse wave velocity and the health of the internal organs can be obtained. As discussed in xe2x80x9cWave Reflection in the Systemic Circulation and its Implications in Ventricular Functionxe2x80x9d, Michael O""Rourke et al., Journal of Hypertension 1993, 11 pgs. 327-337, human aortic pulse wave velocity more than doubles between 17 and 70 years of age. This phenomenon is a manifestation of arterial stiffening and is attributable to the fatiguing effects of cyclic stress causing fracture of load-bearing elastic lamellae in the wall, and degeneration of arterial wall. When mean blood pressure is decreased (i.e. using vasoactive drugs), the reflected wave has been observed to occur later in the pulse wave, whereas when blood pressure is increased, the reflected wave occurs earlier and moves into the systolic part of the wave. Readily observed ascending aortic pressure wave contours associated with ageing and hypertension can be explained on the basis of early wave reflection. Also, several authorities have observed a strong association between poor aortic compliance (i.e. arterial stiffness) and coronary artery disease and hypertension. For example, it has been observed that decreased aortic compliance results in an increase in systolic and a decrease in diastolic aortic pressure, both of which are deleterious to the heart (xe2x80x9cAortic Compliance in Human Hypertensionxe2x80x9d, Zharorong Liu, et al., Hypertension Vol. 14, No. 2, August 1989 pgs. 129-136). Accordingly, the aortic reflected wave is a powerful source of information relating to a user""s cardiovascular health and relative risk.
While there are several techniques for utilizing the timing of the aortic reflected wave to derive physiologically useful parameters, the analysis used by most of these techniques does not accurately identify the onset of the reflected wave in the volume pulse contour. The subtle changes in the volume pulse signal associated with aortic reflection effects that follow the systolic peak are difficult to visualize. It is often extremely difficult to identify these effects, even with the help of computing means, without time consuming pattern recognition techniques.
For example, U.S. Pat. No. 5,265,011 to O""Rourke discloses a method for determining the systolic and diastolic pressures based on the specific contours of pressure pulses measured in an upper body peripheral artery. The method identifies pressure pulse peaks relating to systolic and diastolic components of the pulse contour and takes first and third derivatives of the pressure pulses to determine relevant minimum and maximum points. Specifically, the onset of the systolic pressure wave is determined by locating a zero crossing from negative-to-positive on a first derivative curve and the shoulder of the reflected wave is identified by finding the second negative-to-positive zero crossing on the third derivative. However, it is difficult in practise to identify the reflected wave peak in this fashion as the slope changes of the third derivative do not consistently indicate the reflected wave peak. In addition, this method identifies only slope changes in the blood volume pulse contour. These slope changes are an indirect and imprecise way of characterizing the timing of the reflected wave component. The high degree of overlap between the systolic, reflected and dicrotic wave components obscures the characteristics of the reflected wave.
Also, many established methods that use PPG techniques and volume pulse contour analysis and/or pulse wave velocity to derive blood pressure do not adequately take into account other complicating effects. For example, the volume pulse contour varies with changes in blood volume that are unrelated to blood pressure. Changes in temperature, respiration and body position can all lead to changes in local blood volume. Movement of a finger relative to the sensor will also result in unreliable PPG readings. Unless these factors are controlled, erroneous blood pressure readings will result.
Various established methodologies such as the one disclosed in U.S. Pat. No. 5,876,348 to Sugo et al., derive blood pressure measures on the assumption that pulse wave velocity and blood pressure are linearly related. Specifically, in U.S. Pat. No. 5,876,248 mean blood pressure is derived using the formula P=xcex1 PWV+xcex2, where P is mean pressure, PWV is pulse wave velocity and xcex1 and xcex2 are constants specific to a user. The formula P=xcex1 PWV+xcex2 assumes that the relationship between blood pressure and PWV is linear, which is incorrect. Although the increase in pulse wide velocity is linear for low pressures, authorities confirm that the increase is nonlinear with pressure above typical diastolic pressure (xe2x80x9cMeasurement of Pulse-Wave Velocity Using a Beat-Sampling Techniquexe2x80x9d, J. D. Pruett, Annals of Biomedical Engineering, Vol. 16, pgs. 341-347). Further, the relationship between the excursion of the digital blood volume contour and the arterial pulse pressure is also nonlinear. Current volume pulse contour analysis techniques do not take these considerations into account and result in unreliable determinations.
Accordingly, there is a need for an improved physiological characteristic testing device which provides for improved estimation of various cardiovascular and respiratory indices through the correct identification of the aortic reflected wave and arterial blood pressure which facilitates improved communication of information and biofeedback functionality, uses a minimum of processing and memory capacity, comprises relatively few parts, and which is inexpensive to manufacture and operate.
It is therefore an object of the present invention, to provide a physiological signal monitoring system comprising:
(a) a sensor adapted to come into skin contact with a user body part, for sensing a physiological characteristic of the user and for generating electrical signals which correspond to said physiological characteristic;
(b) first processing means operatively coupled to said sensor for receiving and converting said electrical signals into data, for computing a set of physiological parameters on the basis of said data, said processing means also being operatively coupled to a communication network for transmission of said physiological parameters over said communication network;
(c) display means coupled to said first processing means for displaying said physiological parameters; and
(d) a server coupled to said communications network for receiving said physiological parameters from said processing means, for conducting analysis of said first physiological parameters, and for transmitting information related to said physiological parameters to said first processing means for display on said display means.
In another aspect the invention provides a method of monitoring the physiological signals of a user comprising the steps of:
(a) positioning a sensor in close proximity to a body part of the user for sensing a physiological characteristic of the user and for generating electrical signals which correspond to said physiological characteristic;
(b) receiving and converting said electrical signals into data and computing a set of physiological parameters on the basis of said data;
(c) displaying said physiological parameters to the user;
(d) transmitting said physiological parameters to a server over a communications network; and
(e) analyzing said physiological parameters on said server and transmitting information associated with said physiological parameters to the user.
In another aspect the invention provides physiological signal monitoring system for determining a number of physiological parameters for a user, said monitoring system comprising:
(a) a PPG sensor adapted to come into skin contact with the user for obtaining the blood volume contour of the user;
(b) filtering means for filtering nonpulsatile and slowly pulsatile signals from the blood volume contour to obtain a filtered blood volume pulse signal; and
(c) processing means for extracting a representation of the aortic reflected wave contour from the user""s filtered blood volume pulse signal and for determining a plurality of physiological parameters based on characteristics of said aortic reflected wave.
In another aspect the invention provides a method of determining a number of physiological parameters for a user, said method comprising the steps of:
(a) obtaining the blood volume contour of the user using a first PPG sensor coupled to the user""s body, said blood volume pulse contour containing a plurality of individual blood volume pulse contour pulses;
(b) filtering nonpulsatile and slowly pulsatile signals from the blood volume pulse contour to obtain a filtered blood volume pulse signal;
(c) extracting an estimate of the aortic reflected wave contour from the filtered blood volume pulse signal; and
(d) determining a plurality of physiological parameters based on characteristics of said aortic reflected wave.
The invention also provides a method of determining the systolic and diastolic blood pressure of a user, in addition to the steps of determining a number of physiological parameters for a user described above, comprising the additional steps of:
(e) performing a series of calibration photolethsympographic measurements using said first PPG sensor coupled to the skin of the user over a predetermined calibration period of time;
(f) performing a series of calibration blood pressure measurements of the user using a blood pressure monitor coupled to the user over said predetermined calibration period of time;
(g) determining at least one transfer function which relates said calibration blood volume measurements and said calibration blood pressure measurements;
(h) calculating a synthesized blood pressure pulse contour, RADIALsynth, mean arterial blood pressure, MEANABP, and synthesized pulse pressure, PPsynth, by applying said at least one transfer function to various indices of said user""s blood volume pulse contour obtained from step (a);
(i) determining the pulse pressure of the synthesized blood pressure pulse contour, PP RADIALsynth from said synthesized blood pressure pulse contour, RADIALsynth;
(j) calculating the mean amplitude of the synthesized blood pressure pulse contour, RADIALsynth, namely, MEAN AMP RADIALsynth;
(k) calculating the mean fractional amplitude, MEAN AMPFrac, of said synthesized blood pressure contour, RADIALsynth, according to the relation: MEAN AMPFrac=MEAN AMP RADIALsynth/PP RADIALsynth; and
(l) calculating systolic blood pressure, BPsys, according to the relation: BPsys=MEANABP+PPsynth (1xe2x88x92MEAN AMPFrac).
In another aspect, the invention provides a method of determining the pulse wave velocity of a user, in addition to the steps of determining a number of physiological parameters for a user described above, comprising the additional steps of:
(e) performing steps (a) and (b) using said first PPG sensor coupled to said user""s body at a first location a and a second PPG sensor coupled to said user""s body at a second location b, to obtain a first filtered blood volume pulse signal at location a and a second filtered blood volume pulse signal at location b;
(f) high pass filtering said first and second filtered blood volume pulse signals;
(g) performing cross correlation to obtain the time delay between said first and second filtered blood volume pulse signals according to the relation:       CC    ⁡          (      τ      )        =            ∫              +        ∞              ⁢                            V          a                ⁡                  (          t          )                    ⁢                        V          b                ⁡                  (                      t            -            τ                    )                    ⁢              xe2x80x83            ⁢              ⅆ        t            
where CC(xcfx84) is the cross correlation which depends on the time delay between two parameters Va and Vb; Va(xcfx84) and Vb(xcfx84) are the corresponding values of the first and second filtered blood volume pulse signals at the two different sites on the user""s body, a and b, at a time t, and xcfx84 is the time delay;
(h) estimating the travel path for the user; and
(i) estimating the user""s pulse wave velocity on the basis of said time delay and said travel path.
In another aspect the invention provides a method for the extraction of a respiration contour from said blood volume pulse, in addition to the steps of determining a number of physiological parameters for a user described above, comprising the additional steps of:
(e) calculating an indicia based on said blood volume pulse contour that correlates with the mean blood pressure of the user;
(f) plotting the amplitude values of said indicia over time;
(g) interpolating said amplitude values over time to obtain an interpolated respiratory contour; and
(h) low pass filtering the interpolated respiratory contour to obtain the respiration contour.
In another aspect the invention provides a method for temperature correcting a user""s blood volume pulse contour, in addition to the steps of determining a number of physiological parameters for a user described above, comprising the additional steps of:
(e) artificially lowering the temperature of the user""s finger prior to step (a) and conducting step (a) as said finger increases in temperature;
(f) determining the amplitude of the blood volume pulse contour and the amplitude of the filtered blood volume pulse signal at a plurality of sample times, N;
(g) calculating the changes in amplitude of the blood volume pulse contour, xcex94PPG, and changes in amplitude of the filtered blood volume pulse signal, xcex94DVP, over said plurality of sample times, N;
(h) calculating a plurality of constants, Ki for i=Nxe2x88x921 sample times where Ki=xcex94PPG/xcex94DVP;
(i) averaging the values of said plurality of constants Ki to obtain temperature constant K; and
(j) using said temperature constant K to calibrate readings of said filtered blood volume pulse signal by using the relation: xcex94PPG=Kxcex94DVP.
In another aspect the invention provides a method of determining a correlate for said plurality of physiological parameters, in addition to the steps of determining a number of physiological parameters for a user described above, comprising the additional steps of:
(e) twice differentiating said filtered blood volume pulse signal to produce a second derivative;
(f) providing a horizontal axis for indicating time with said second derivative extending above and below said horizontal axis and located on the horizontal axis at the start of said second derivative for each pulse; and
(g) determining the ratio of the height of the second peak above a first trough of said second derivative relative to the height of the horizontal axis above the first trough of said second derivative of said filtered blood volume pulse signal.
In another aspect the invention provides a manually operated user input device for simultaneously sensing a physiological characteristic of a user and for providing input of data unrelated to the physiological characteristic, said device comprising:
(a) a housing having a surface in at least intermittent contact with a portion of the user""s finger;
(b) at least one PPG sensor disposed on said surface for sensing the physiological characteristic of the user; and
(c) manually operated: means for inputting data to the user input device, said data unrelated to the physiological characteristic.
In another aspect the invention provides a device for removable attachment to an extremity of the body of a user, said device comprising:
(a) a housing having a surface in at least intermittent contact with a portion of the user""s extremity;
(b) at least one PPG sensor disposed on said surface for sensing the physiological characteristic of the user; and
(c) biasing means coupled to said housing for holding said portion of said extremity against said PPG sensor with constant and predictable pressure and for shielding said PPG sensors from ambient light.